Embodiments of the present invention relate to data decoding-decompression, particularly it relates to a method for decoding-decompressing digital images.
Numerous algorithms are currently known for compressing and encoding both single (or still) digital images, used for example in photographic applications, and digital image sequences, used for example in video applications.
The compression-encoding algorithms make it possible to reduce the quantity of memory required for the storage of the single images or video sequences. These algorithms further make it possible to reduce the bandwidth resources required for the transfer of the images between different devices or for their transmission on telecommunications networks such as, for example, the Internet.
A compression-encoding algorithm transforms a digital image, or a digital video sequence, into a compressed and encoded digital data sequence, such as in the form of a bit sequence (or bit stream).
The most common and efficient compression-encoding methods currently used are based on an operation of transforming the images into a bidimensional spatial frequency domain. Amongst these, numerous methods use the Discrete Cosine Transform (DCT). One example thereof are the compression-encoding methods, for single images, compliant with the international JPEG (Joint Photographic Experts Group) standard and the compression-encoding methods, for video sequences, compliant with the international MPEG (Motion Picture Experts Group) standard such as MPEG-1, MPEG-2, MPEG-4.
Amongst the other standards that use compression-encoding methods based on the DCT transform, we further mention the standards H263 and H26L.
The transformation operation is followed by a subsequent processing, which reduces the informative content of a digital image or a digital image sequence, by operating directly in the spatial frequency domain.
After this processing aimed at reducing the informative content, the images are encoded according to known algorithms, through methods of entropic coding, commonly of the type including a variable length coding (VLC). Huffman coding is, for example, a particular type of entropic variable length coding (VLC) which reduces the number of bits necessary to represent a data set without introducing any loss of information.
The thus compressed and encoded images, or video sequences, are transferred in the form of a bit sequence into storage devices or are remote transmitted, for example they are exchanged between multimedia communication terminals.
The decoding-decompression process, typically inverse to that of compression-encoding, is aimed at the reconstruction of the digital images or video sequences from the compressed and encoded data sequence. For example, the decoding-decompression process is used in applications that require either such digital images or such video sequences to be displayed on a screen.
It is known that a bit sequence, during storage or transmission, can be corrupted, that is, altered by errors.
This problem is particularly felt when the bit sequence is transmitted on radio channels, such as those used in mobile telecommunications and video-communications. The presence of disturbances of various kinds on the channel may vary the value of some bits in the sequence, which may introduce errors.
The decoding-decompression performances of an encoded-compressed bit sequence with methods that use an entropic variable length coding (VLC) are sensitively influenced by the presence of possible errors that have corrupted the bit sequence to be decoded.
This is due, for example, to the fact that certain errors can be such that a code word is erroneously interpreted as another code word with a different length, without the presence of an error being detected in the decoding step. This condition may determine a loss of synchronization in the decoding step.
In this way, even an error in a single bit may entail the loss of a large quantity of data thus producing a significant degradation in the quality of the video or the image.
It has been observed that the state of the art decoding-decompression techniques, for example those compliant with the different standards currently existing, in the presence of errors that alter the compressed-encoded bit sequence, do not ensure satisfactory performances in terms of the quality of the decoded-decompressed image or sequence.